Heretofore, PDP is generally produced by firing metal electrodes, an insulating paste, a rib paste, etc. on a substrate glass at a temperature of from about 550 to 600° C., followed by frit sealing opposed plates and its vicinity. Heretofore, in general soda lime silica glass which is widely used for buildings and automobiles has been used as a glass for a PDP substrate.
However, since the glass transition temperature of the soda lime silica glass is from 530 to 560° C., if the glass is heat-treated at the above firing temperature, the substrate glass deforms or shrinks, and the dimension remarkably changes. Therefore, it has been desired to realize positioning opposed plates and electrodes accurately. Particularly, in a case where a continuous furnace such as a belt furnace, of which productivity is high, is used, there is a problem that due to the temperature difference between the anterior and the posterior of the substrate glass, a dimensional change results such that the substrate glass becomes asymmetrical at anterior and posterior. Such a problem is remarkable in the case of a large substrate glass, for example, a 40-inch size PDP, and a substrate glass having a higher heat resistance is required.
In order to solve the problem of the heat deformation or the heat shrinkage of the substrate glass, a glass for a display substrate has been known, wherein the thermal expansion coefficient is near to that of soda lime silica glass, and the glass transition temperature and the distortion point are high (Patent Documents 1 and 2).
If such a glass is used as a glass for a PDP substrate, when the heat treatment for producing PDP is carried out in a continuous furnace, the anterior-posterior asymmetrical dimensional change, which is the problem in the soda lime silica glass, can be prevented, whereby a substrate glass can be fired highly accurately.
However, compared to the soda lime silica glass, in the glasses for display substrates described in Patent Documents 1 and 2, the viscosity of the glasses at a high temperature (for example, the viscosity of the glasses at a temperature of at least 700° C.) is high. Therefore, it is required to increase the temperature at each step carried out for producing a substrate glass, namely it is required to increase the glass melting temperature, the glass clarification temperature and the glass forming temperature, and problems result such that stable production is difficult, life span of production apparatuses is harmed, and production cost of a substrate glass is high.
Thus, it is required to lower the viscosity of glass at a high temperature for stable production of glass, the extension of life span of production apparatus, cost reduction, etc.
However, in the case of the glass composition, of which the high temperature viscosity is simply lowered, properties required for the substrate glass for PDP cannot be satisfied. Namely, properties such as the specific gravity of the substrate glass, the glass transition temperature, chemical resistance, dielectric constant, volume resistivity, reflectance, thermal expansion coefficient and distortion point cannot be satisfied within the range required for the substrate glass for PDP.
Further, in the case of the glass composition, of which the high temperature viscosity is simply lowered, the temperature of each step carried out for producing the substrate glass can be thereby lowered, however, if the temperature at the clarification step is lowered, the effect of a clarifier deteriorates. At the time of producing the glass for a display substrate, SO3 is usually used as a clarifier to reduce foams (bubbles) in the glass. SO3 in the form of a sulfate of an alkaline earth metal is added in a glass material. Then, SO3 decomposes in molten glass, whereby SO2 is generated. This SO2 grows bubbles present in molten glass, whereby the clarification effect is improved. However, if the temperature at the clarification step is lowered in accordance with the composition, of which the high temperature viscosity is lowered, SO3 is not likely to decompose in molten glass, and the effect of the clarifier deteriorates.
Patent Document 1: JP-A-3-40933
Patent Document 2: JP-A-8-165138